Current when I choose a photo to be set as wallpaper, my lock screen's background is automatically changed to be this new one.
I want the lockscreen to keep its old background, how can I do that?
I'm using Google Samsung Galaxy Nexus.
Although I am on another device with another android version (HTC Desire HD / ROM: JellyTime R9, Android 4.2.2) I think the problem may be the same.
In the Gallery app I can only set the images as background. But I can set a different lock screen background image in system settings. I think this option depends on the specific Launcher/Lockscreen you use and are also valid in stock roms. I am using NovaLauncher and I can only set a different lockscreen image that way as far as I know.
Look in Settings → UI → Lock screen → Background → Custom image (maybe different wording — I only have the German version here).
This is what you need to do, supposing you're on a stock ROM:
If you have a custom ROM, you should check the forums of that ROM to see if its supports this.
You can't do that. But what you can do is download a third party locker such as GO Locker.
I had a bit of trouble setting the lock screen pic. I have a Samsung Galaxy Ace Q i827 (Appeal in the U.S.) & I had to first find a pic I wanted, then I had to hit the menu button a couple of times to find the 'set as' button. At first I only hit it once, & no 'save as' button appeared. But after hitting it again, it popped up. After clicking on the 'set as', I had the option of selecting the lock screen or wallpaper. This is for gingerbread 2.3.6.